In the prior art, a PLC of the type described in the introduction now carries out a data transaction with a server online, that is to say during continuous operation, for example during the control of a machine. This data transaction is initiated by the PLC, which establishes a TCP/IP connection to the appropriate server. The resulting network process is supported on the PLC side by a corresponding IEC program and on the server side by the associated process. The protocol that is used is the ISOonTCP protocol. The server receives the transaction request, carries out the appropriate operations and sends the transaction response back again to the PLC. The request for data (such as article number or serial number, for example) can be considered as a transaction, just like the transmission of data where process results are to be assigned to a serial number, for example.
The problem with such a procedure is the outlay involved with the creation of the PLC programs to execute the transactions. Among other things, what is important is that very close matching of the two programs is necessary as the implementation of new functions and modifications has to be effected in a detailed manner, both at the transmit end and at the receive end (at the client).
For this reason, a further option for executing such transactions has emerged in the prior art. Web services are used for this.
In principle—in comparison to web pages—web services are not intended for human users, rather for software systems which exchange data automatically and/or initiate functions on remote computers. In this case a web service is a software application which can be uniquely identified by a uniform resource identifier (URI) and is thus addressable and whose interfaces are defined as XML artifacts. A web service supports direct interaction with other software agents by using XML-based messages thru exchange via Internet-based protocols. Requests to a web service are made by client programs and the web service responds with the desired information.
A method, in which a PLC as client communicates with a web service, is known from U.S. Pat. No. 6,915,330. Here there is a problem in that the function blocks describing a web service, as well as the function blocks activating the web service, have to be specially realized for this web service, which in practice means a large outlay.